The crystal structure of the mineral mertieite-II from the Kaarreoja River, Inari commune, Finnish Lapland, Finland, was refined to R1 = 0.0222 (I) and 0.0228 (II) on the basis of X-ray diffraction data collected from two single crystals. The mineral is trigonal, space group is $R\bar 3c$. The unit-cell parameters for the two crystals are determined as: a = 7.5172(3), c = 43.037(2) Å, V = 2106.1(2) Å3 (I); a = 7.5135(4), c = 43.003(3) Å, V = 2102.4(3) Å3 (II) with Z = 12. The occupancies of the Sb and As position in the structure were refined according to the Sb:As ratio of each crystal. The position As1 (Wyckoff 6b) is completely filled by As atoms. Any excess of As is distributed together with Sb on structural position M1 (Wyckoff 12c). The crystal chemical formulae are defined as Pd8Sb1.5(Sb0.94As0.06)As0.5 (crystal I) and Pd8Sb1.5(Sb0.88As0.12)As0.5 (crystal II). As → Sb substitution of up to 4.50 wt% of As does not affect the main structural topology of mertieite-II. Mertieite-II, Pd8Sb2.5As0.5 and synthetic Pd8Sb3 are isotypic compounds. The crystal structures of synthetic Pd8Sb3 and natural mertieite-II, Pd8Sb2.5As0.5, can be derived from the hexagonal close packing by filling additional layers between the close-packed 36 layers. The structure consists of Sb and (Sb,As) triangular 36, Pd triangular 36 and Pd pentagonal-triangular 5.33 layers. Stacking of Sb-, (Sb,As)- and Pd-nets along the z axis caused extension of the unit cell. It contains a total of 36 layers: six Sb1-nets, six (M1,As1)-nets, 12 Pd1-nets and 12 (Pd1,Pd3,Pd4)-nets.
A number of experiments have been carried out using Cl-, Na-, K-, As-, Sn-, Bi-, Pb-, Te-bearing solutions at temperatures between 240 and 500•C in order to synthesize platinum-group minerals (PGM) and to study their relationships under hydrothermal conditions. Metallic platinum, palladium, PtAs2, Pt(As,Sn)2, and PdPb ("Pb-polarite") were synthesized directly from hydrothermal solutions in the presence of graphite as reducing component. Phase transformations were observed under hydrothermal conditions for some pre-synthesized Pd compounds: Pd bismuthides replace PdS in CI-Sb-Bi-bearing solutions. However, no Pt-Sb or Pt-Bi phases crys tallized under these conditions. Recrystallization of Pt-and Pd-sulphides in slightly acidic solutions with Sb-or Bi-chlorides at 240"C yielded complex multiphase associations including Sb2S3, BiSb3 and BhS3. The reactions leading to their formation did not include any platinum. However, froodite (PdBh) was obtained by recrystalliza tion of PdS + Feo.ssS + FeS2 assemblages in BiCb solution at 240•C. Experiments to investigate if PGE-bearing sulphides can be transformed to sulpharsenides by recrystallization gave negative results. Platinum-group elements (PGE) redistribution under hydrothermal conditions at controlled sulphur fugacity shows that PtS is stable at logfS2 >-3.4 and (Pt,Fe) is very poor in Fe at logfS2 <-6.5 (at 562•C). Experimental data obtained for some PGE-phases (Pt/Pd-and Ir-bearing sulphides) are in good agreement with observations on natural deposits.
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